Compensation for differences in gain among amplifiers

ABSTRACT

A method and apparatus for measuring the small signal gain of an amplifier. Each amplifier responds, in operation, to a small signal input. The small signal input varies substantially linearly over a field time interval. A small bias signal is added to the bias level input of the amplifier during a portion of the field time interval. The output of the amplifier is measured at three or more different times during the field time interval, at least one time when the small bias signal has not been added to the bias level input and at least one time being when the small bias signal has been added to the bias level input. By processing the measured outputs, a signal which is directly proportional to the product of the small bias signal with the small signal gain of the amplifier is obtained. A method and apparatus for actively compensating for differences in the small signal gain among two or more amplifiers is obtained by dividing the small signal output of each amplifier by the signal proportional to the small signal gain of the amplifier.

The U.S. Government has rights in this invention pursuant to ContractNo. DAAK 70-78-C-0011 awarded by the U.S. Army Night Vision andElectro-Optics Laboratory.

This is a division of application Ser. No. 358,541, filed Mar. 15, 1982,now U.S. Pat. No. 4,481,477.

BACKGROUND OF THE INVENTION

The invention relates to a method and apparatus for actively measuringthe small signal gain of an amplifier. That is, the invention relates toa method and apparatus for periodically measuring the small signal gainof an amplifier while the amplifier is in operation. The invention alsorelates to a method and apparatus for actively compensating fordifferences in the small signal gain among two or more amplifiers.

In recent years, much work has been performed on the development ofsolid state imaging devices. These devices have been proposed in manyforms, depending upon the requirements of the particular applicationinvolved. In the field of solid state infrared imaging devices, a devicehas been proposed and built in which an X-Y addressable array ofjunction field effect transistors (JFET's) is provided with apyroelectric target material. This device is disclosed in an articleentitled "Solid-state pyroelectric imaging system" by A. Carlson et al(Proceedings of SPIE, Vol. 267 Staring Infrared Focal Plane Technology,1981).

The use of an array of JFET's is advantageous, among other reasons,because it greatly reduces amplifier noise. This is achieved becauseJFET's are inherently low-noise devices, and because an array of JFET'scan provide individual amplification of each pyroelectric channel beforemultiplexing. However, in the use of a JFET array, other problems arise.These problems are the nonuniformities in the characteristics of theindividual JFET's (or other amplifiers) in the array. There are twokinds of nonuniformities, namely offset and gain. While offsetnonuniformities are readily corrected in the known device, there is nomethod suggested in this article for correcting gain nonuniformities.

SUMMARY OF THE INVENTION

It is an ojbect of the invention to provide a method and apparatus foractively measuring the small signal gain of an amplifier.

It is another object of the invention to provide a method and apparatusfor compensating for differences in the small signal gain among two ormore amplifiers.

It is yet a further object of the invention to provide a method andapparatus for actively compensating for the differences in the smallsignal gains among two or more amplifiers, while the amplifiers are inoperation.

According to the invention, a method and apparatus are provided formeasuring the small signal gain of an amplifier which, in operation,responds to a small signal input variation about a bias level input. Thesmall signal input varies substantially linearly over each field timeinterval.

The method according to the invention is performed by adding a smallbias signal, ΔV, to the bias level input during a portion of the fieldtime interval. The output of the amplifier is then measured at a firsttime during the field time interval when the bias signal, ΔV has notbeen added to the bias level input. This produces a first measuredoutput.

The output of the amplifier is also measured at a second time during thefield time interval to produce a second measured output. The biassignal, ΔV has been added to the bias level input at the second time.Moreover, the output of the amplifier is measured a third time toproduce a third measured output. The third time is different from thefirst and second times, but it does not matter whether or not the biassignal, ΔV, has been added to the bias level input at the third time.

According to the invention, each of the three measured outputs ismultiplied by a respective preselected fixed number to produce weightedmeasured outputs. These weighted measured outputs are then added in sucha way that the sum is a signal which is directly proportional to theproduct of the bias signal, ΔV, with the small signal gain of theamplifier during the field time interval. (As used herein, an outputsignal is "directly proportional" to the small signal gain when theoutput signal is equal to the small signal gain multiplied by one ormore constant factors.)

The method of active compensation according to the invention isperformed by first producing, by the preceding method, a signal which isdirectly proportional to the small signal gain of each amplifier.Compensation is then achieved by dividing the small signal output ofeach amplifier by the signal proportional to the small signal gain ofthe amplifier. This process is repeated for each and every field timeinterval while the amplifiers are in use. Consequently, the process notonly compensates for differences in gain between amplifiers, but it alsocompensates for changes in the gain of a single amplifier over time.

An apparatus for measuring the small signal gain of an amplifier duringoperation of the amplifier includes means for adding a small biassignal, ΔV, to the bias level input during a portion of the field timeinterval. Such an apparatus further includes means for measuring thefirst, second and third measured outputs, described above, and means formultiplying each measured output by the respective preselected fixednumbers to produce weighted measured outputs. Finally, the apparatusincludes means for adding the weighted measured outputs to produce asignal which is directly proportional to the small signal gain of theamplifier. Digital electronics can be easily designed to perform thesefunctions.

An apparatus according to the invention for active gain compensationincludes the apparatus for measuring the small signal gain of eachamplifier, as well as means for dividing the small signal output of eachamplifier by the signal proportional to the small signal gain of theamplifier. Either an analog or digital divider can perform this latterfunction.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a graph showing an approximation of the small signal input tothe amplifier whose gain is being measured according to the invention.

FIG. 2 is a graph showing the total bias signal applied to theamplifier, which is the sum of the bias level input and the small biassignal, ΔV.

FIG. 3 is a schematic diagram showing the flow of data according to theinvention from the output of the amplifier to both a gain-proportionalsignal and a gain-compensated video signal.

FIG. 4 is a schematic representation of an array of JFET's whose outputsare fed into an analog shift register.

FIG. 5 is a series of graphs showing the relative timing between theshutter (chopper), the signal applied to the first control gate line,and the pyroelectric voltage of the pyroelectric target material.

FIG. 6 is a schematic representation of a JFET pyroelectric imagingarray provided with gain compensation according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The method of measuring the small signal gain of an amplifier inreal-time is based upon measuring this gain once during each field timeinterval. The field time interval may be any preselected length of time.However, the effectiveness of this method is based upon the assumptionthat during the field time interval, the small signal input to theamplifier varies substantially linearly. The closer the small signalinput approximates a linear variation, the more accurate will be thesmall signal gain measurement during the field time interval.

FIG. 1 shows a small signal input which varies linearly over a fieldtime interval. The field time interval is shown divided into four equalsegments. The slope of the curve shown in FIG. 1, which is the change inthe small signal input with time, is denoted by the function S(t). Aswill be discussed further below, the small signal generated by apyroelectric target material can be made to closely approximate astraight line over a field time interval.

FIG. 2 is a graph showing a typical total bias signal applied to theamplifier in order to measure the amplifier's gain. Preferably, duringone half of the field time interval, the total bias signal is equal tothe bias level input. During the other half of the field time interval,the total bias signal is equal to the sum of the bias level input,V_(b), and the small bias signal, ΔV.

Where the amplifier is a JFET, the small signal input and the total biassignal are both voltage signals. However, in general they may be anyform of electrical signal.

In order to obtain a signal which is directly proportional to the smallsignal gain of the amplifier during the field time interval, it ispreferred that four measurements of the output of the amplifier aretaken during the field time interval. Each measurement is taken in themiddle of each quarter of the field. Thus, each of the four measuredoutputs will be given by the following equations:

    I(t.sub.o)=I.sub.o

    I(t.sub.o +Δt)=I.sub.o +g.sub.m [ΔV+S(t)Δt]

    I(t.sub.o +2Δt)=I.sub.o +g.sub.m [ΔV+2S(t)Δt]

    I(t.sub.o +3Δt)=I.sub.o +g.sub.m [3S(t)Δt]

In these equations, I_(o) is the amplifier's output due to the biaslevel input (and possibly leakage). The quantity g_(m) represents thesmall signal gain of the amplifier during the field time interval. Thesymbol ΔV represents the small bias signal, which is added to the biaslevel input during the second and third quarters of the field timeinterval. The symbol Δt represents the time interval between eachmeasurement. This time interval is equal to one fourth of the field timeinterval in the preferred embodiment of the invention. Finally, S(t)represents the rate of change of the small signal input with time, asdiscussed above.

Now, the gain-proportional output signal can be obtained by adding thesecond and third measured outputs (each multiplied by +1) andsubtracting from this sum both the first and fourth measured outputs(i.e. adding the first and fourth measured outputs, each multiplied by-1).

    I(t.sub.o +Δt)+I(t.sub.o +2Δt)+(-1)I(t.sub.o)+(-1)I(t.sub.o +3Δt)=2g.sub.m ΔV (gain-proportional signal)

The resulting output signal is equal to two times the small signal gainduring the field time interval multiplied by the small bias signal, ΔV.Since the small bias signal is known, the small signal gain can easilybe obtained from this output signal.

While the preferred method of the invention makes use of fourmeasurements during each field time interval, it should be readilyapparent from the above discussion that only three measurements arenecessary in order to obtain a signal which is directly proportional tothe small signal gain. This follows from the fact that each measuredoutput includes components which make up three unknowns, namely thesignal component due to the bias level input (and possibly leakage), thesmall signal input itself, and the small signal gain during the fieldtime interval.

For example, a gain-proportional signal, g_(m) ΔV can be obtained bymultiplying the second measured output by two and then adding to thisboth the first and third measured outputs, each multiplied by -1.

    2[I(t.sub.o +Δt)]+(-1)I(t.sub.o)+(-1)I(t.sub.o +2Δt)=g.sub.m ΔV

When the measurements are in the form of digital signals, as ispreferred, a multiplication by two is performed by a simple "arithmeticshift left" operation. In order to multiply by -1 digitally, a simple"2's complement" operation is performed.

Moreover, the three measurements taken during the field time intervalneed not be separated by equal times, Δt. So long as the time betweenmeasurements is known, the gain-proportional signal can be obtainedmerely by the selection of an appropriate multiplier for eachmeasurement. For similar reasons, the small bias signal, ΔV, may beadded to the bias level inputs at any time during the field timeinterval.

It is important, however, that at least one measurement is taken whenthe total bias signal is equal to the bias level input, and that anothermeasurement is taken when the total bias signal is equal to the sum ofthe bias level input plus the small bias signal, ΔV. Absent thisrequirement, there would be no way to obtain the value of the smallsignal gain according to the invention.

FIG. 3 is a schematic diagram showing the flow of measurement dataaccording to the method and apparatus of the invention. This flowapplies to the data from each amplifier when an array of amplifiers isused. First, the output from the amplifier is continuously fed into ananalog-to-digital converter. While this is proceding, a scanner (clock)is sequentially feeding address signals into an address bus. In thisway, each time the output of the amplifier is measured during a fieldtime interval, either the value of the output itself or the result ofsome arithmetic operation performed on the output can be stored in amemory location defined by a unique address. In order to performarithmetic operations on the supplied data, an arithmetic logic unit anda register are provided. The arithmetic logic unit is preprogrammed toperform the following sequence of operations in order to calculate thegain-proportional signal when four measurements are taken according tothe preferred embodiment discussed above.

At the time of the first measurement, the output from theanalog-to-digital converter (ADC) is transferred to either memory A ormemory B (depending on whether the field is "even" to "odd").

Next, at the time of the second measured output, the first measuredoutput from the current field (i.e. the contents of memory A or B) istransferred to the register. The second measured output from theamplifier is then read and fed into the ADC. The value of the contentsof the register is subtracted from the value of the output of the ADC,and this difference is stored in memory D.% Thus, memory D now containsthe second measured output minus the first measured output.

At the time that the third measured output is taken, the contents ofmemory D are transferred to the register. The third measured output isthen added to the register and the sum is then transferred to memory D.now, memory D contains the sum of the second and the third measuredoutputs minus the first measured output.

At the time of the fourth measured output, the contents of memory D areagain transferred to the register. Next, the value of the output of theADC (i.e. the fourth measured output) is subtracted from the contents ofthe register to produce the signal equivalent to 2g_(m) ΔV. The value ofthis signal is then stored in memory E.

Just before or after the operations described in the precedingparagraph, the contents of memory A or B (depending on the field beingeven or odd) are transferred to the register and subtracted from thefourth measured output. This produces a "video" signal equivalent to3g_(m) S(t)Δt. (This is called a "video" signal because the use of anarray of amplifiers with a pyroelectric imaging system is described,below.) Thus,

    I(t.sub.o +3Δt)-I(t.sub.o)=3g.sub.m S(t)Δt.

This video signal is stored in memory C.

Now, by synchronizing (via the clock) the read-out of memories C and Ecorresponding to the same field time interval, the video signal (C) canbe divided by the gain-proportional signal (E) to produce again-compensated video signal, [3S(t)Δt]/[2ΔV]. If this is performed bya digital division, no additional noise is added to the gain-compensatedvideo signal. If the division is performed by an analog divider, it maybe desireable to digitally "amplify" the video signal (by multiplicationby a constant) prior to conversion back to an analog signal, so as toincrease the signal-to-noise ratio.

While thusfar a method and apparatus have been described for activelymeasuring the small signal gain of an amplifier, the value of such amethod and apparatus can be greatly appreciated when used in connectionwith an array of amplifiers. For example, FIG. 4 schematically shows anarray of junction field effect transistors 10. Each JFET is providedwith a source, a drain, a sense gate, and a control gate. (Forsimplicity, these are not shown.) It can be seen, however, from FIG. 4that in each row of JFET's, each control gate is electrically connectedto a common control gate line. Similarly, for each column of JFET's,each drain is electrically connected to a common drain line. Each drainline is then connected to a multiplexer such as an analog shiftregister, for example a bucket brigade device, via an impedanceconverter for matching the JFET drain impedance to the input impedanceof the shift register.

In the operation of such an array, the control gate lines are energizedsequentially. When one control gate line is energized, the outputs fromall of the JFET's in the row travel in parallel into the shift register.The shift register then serially shifts these outputs to theanalog-to-digital converter. After the shift register is emptied, thenext control gate line is energized, repeating the process.

Such an array of JFET's can advantageously be used in connection with animaging sensor, such as a pyroelectric target material. By providing theamplifiers immediately following the sensor outputs, signal noise can beminimized. However, by providing many individual amplifiers there is ahigh likelihood that there will be variations in the values of the smallsignal gains of the amplifiers. Hence, the method and apparatus fordetermining the value of the gain-proportional signal can be used tocompensate for the gain nonuniformities among the JFET's.

FIG. 6 is a schematic representation of a JFET pyroelectric imagingsystem provided with gain compensation. The system includes a camerahead 12 and an external controller 14. The camera head includes a JFETarray 16 having a pyroelectric target material provided thereon. A lens18 focuses a thermal image onto the pyroelectric target.

As is well-known, pyroelectric materials produce output signals inresponse to changes in the temperature of the target material.Accordingly, a chopper 20 is provided to produce periodic changes in thetemperature of the pyroelectric target by periodically blocking thethermal image from impinging on the target. An optical pickup 22 isprovided for generating a feedback signal for use in synchronizing thechopper 20 with respect to the remainder of the imaging system.

Referring to FIG. 5, there is illustrated a series of graphs showing therelative timings between the chopper, the signal applied to the firstcontrol gate, the total bias, and the pyroelectric voltage of a typicalportion of the pyroelectric target material. The operating cycle is madeup of an open field A and a closed field B as shown in the top portionof FIG. 5. The time period of each field is chosen so that thepyroelectric voltage varies substantially linearly over most of thefield time interval. This is shown in the bottom portion of FIG. 5 wherethe pyroelectric voltage increases substantially linearly during theopen field and decreases substantially linearly during the closed field.The middle portion of FIG. 5 shows two occasions during each field whenthe first control gate line is energized in order to read the outputs ofall JFET's in the first row. The 128 microsecond pulse applied to thecontrol gate at the beginning of each field is a reset pulse; this issimilar to the 64 microsecond reset pulse described in the article by A.Carlson et al.

Returning to FIG. 6, the external controller 14 is provided with a clockor scanner for synchronizing the operations of the chopper, the controlgate drivers, the analog shift register, the analog-to-digitalconverter, the image signal processor (that is, the arithmetic logicunit, register, memories, and programming), and the digital divider. Allof these elements are operated in the manner described in connectionwith FIG. 3. The only difference is that the series of outputmeasurements from each amplifier are multiplexed with each other toproduce a larger series of output measurements from all of theamplifiers in the array. Since each item of data is stored in a memorylocation having a unique address, the output measurements from eachamplifier can easily be segregated from the output measurements of eachother amplifier. For simplicity, each address can simply be taken as thevalue of the time interval between the measurement and the beginning ofthe cycle (one cycle being equal to two fields, an open field and aclosed field).

I claim:
 1. A method of actively compensating for differences in thesmall signal gain among two or more amplifiers, each of said amplifiers,in operation, producing a small signal output in response to a smallsignal input variation about a bias level input, said small signal inputvarying substantially linearly over a field time interval, said methodcomprising the steps of:producing signals which are directlyproportional to the small signal gains of the amplifiers, onegain-proportional signal being produced for each amplifier; and dividingthe small signal output of each amplifier by the signal proportional tothe small signal gain of the amplifier; characterized in that eachgain-proportional signal is produced by a method comprising the stepsof: adding a small bias signal, ΔV, to the bias level input during aportion of the field time interval; measuring the output of theamplifier at a first time during the field time interval to produce afirst measured output, said first time being when the bias signal, ΔV,has not been added to the bias level input; measuring the output of theamplifier at a second time during the field time interval to produce asecond measured output, said second time being when the bias signal, ΔV,has been added to the bias level input; measuring the output of theamplifier at a third time during the field time interval to produce athird measured output, said third time being different from the firstand second times; multiplying each measured output by a respectivepreselected fixed number to produce weighted measured outputs; andadding the weighted measured outputs in such a way that the sum is asignal which is directly proportional to the product of the bias signal,ΔV, with the small signal gain of the amplifier during the field timeinterval.
 2. A method as claimed in claim 1, characterized in that:priorto the dividing step, the small signal outputs and the signalsproportional to the small signal gains are each digital signals; and thedividing step is performed by a digital division.
 3. A method as claimedin claim 1 or 2, characterized in that the bias level input issubstantially the same for each amplifier, and the small bias signal, ΔVis substantially the same for each amplifier.
 4. A method as claimed inclaim 3, characterized in that each first time is earlier than eachsecond time, and each second time is earlier than each third time.
 5. Amethod as claimed in claim 4, characterized in that:each third time is atime when the bias signal, ΔV, has been added to the bias level input;and the method further comprises the step of measuring the output ofeach amplifier at a fourth respective time during the field timeinterval to produce a fourth measured output for each amplifier, each ofsaid fourth times being later than the third respective times and beinga time when the bias signal, ΔV, has not been added to the bias levelinput.
 6. A method as claimed in claim 5, characterized in that:the timeintervals between the measuring steps for each amplifier aresubstantially constant; and the multiplying step comprises multiplyingthe first and fourth measured outputs by -1, and multiplying the secondand third measured outputs by
 1. 7. A method as claimed in claim 6,characterized in that each measuring step is performed sequentially onall amplifiers before the next measuring step has begun.
 8. An apparatusfor actively compensating for differences in the small signal gain amongtwo or more amplifiers, each of said amplifiers, in operation, producinga small signal output in response to a small signal input variationabout a bias level input, said small signal input varying substantiallylinearly over a field time interval, said apparatus comprising:anapparatus for producing signals which are directly proportional to thesmall signal gain of the amplifiers, one gain-proportional signal beingproduced for each amplifier; and means for dividing the small signaloutput of each amplifier by the signal proportional to the small signalgain of the amplifier; characterized in that the apparatus for producingthe gain-proportional signals comprises for each amplifier: means foradding a small bias signal, ΔV, to the bias level input during a portionof the field time interval; means for measuring the output of theamplifier at a first time during the field time interval to produce afirst measured output, said first time being when the bias signal, ΔV,has not been added to the bias level input; means for measuring theoutput of the amplifier at a second time during the field time intervalto produce a second measured output, said second time being when thebias signal, ΔV, has been added to the bias level input; means formeasuring the output of the amplifier at a third time during the fieldtime interval to produce a third measured output, said third time beingdifferent from the first and second times; multiplying each measuredoutput by a respective preselected fixed number to produce weightedmeasured outputs; and means for adding the weighted measured outputs insuch a way that the sum is a signal which is directly proportional tothe product of the bias signal, ΔV, with the small signal gain of theamplifier during the field time interval.
 9. An apparatus as claimed inclaim 8, characterized in that the apparatus further comprises:ananalog-to-digital converter for converting the small signal outputs ofthe amplifiers into digital signals; and a digital divider for dividingthe small signal outputs by the respective signals proportional to thesmall signal gains.